1. Field of the Invention
The present invention relates to a fluorescent lamp having a reflective layer, and more specifically to a fluorescent lamp having two fluorescent layers partially laminated on the inner wall of a glass tube as well as to a method of fabricating the same.
2. Description of the Prior Art
Ordinary fluorescent lamps have a single phosphor 2 which is substantially uniformly coated on the entire surface of the inner wall of a glass tube 1 as cross-sectionally illustrated in FIG. 1. With such conventional fluorescent lamps, nearly a uniform light distribution can be obtained in the radial direction from the center of the glass tube 1. In practice, however, the luminous flux in the main illuminating direction which is a particular radial direction from the center of the glass tube 1, is utilized. To meet the above purpose, there has been proposed a fluorescent lamp having a reflective layer. As cross-sectionally illustrated in FIG. 2, this type of fluorescent lamp has a reflective coating 3 of titanium oxide on the inner wall of the glass tube 1 on the side opposite to the main illumination direction, and a coating 4 of phosphor formed on the entire inner surface of the glass tube, such that the luminous flux generated in the glass tube 1 is radiated in a concentrated manner through a light-transmitting surface on the lower side of the glass tube 1 to obtain strong illumination intensity beneath the lamp.
There has also been known in the art a lamp which uses apatite as the reflective coating 3 instead of the abovementioned titanium oxide. The apatite absorbs ultraviolet rays in very much smaller amounts than titanium oxide, enabling the luminous efficiency of the lamp to be enhanced. There has also been known in the art a lamp which uses the phosphor itself as the reflective coating 3 in place of titanium oxide. This art is based on the fact that the phosphor absorbs ultraviolet rays to a smaller degree than the titanium oxide contributing to the increase in the luminous efficiency of the lamp, and that the phosphor for forming the reflective coating serves as a phosphor that will be coated on the entire surface in a subsequent step thereby to provide convenience for the manufacturing steps. In all of the above-mentioned conventional lamps, the reflective coating has generally been formed on the inner surface of the glass tube 1 at a predetermined reflecting angle .theta., for example, at 230 degrees along the axial direction.
However, the conventional fluorescent lamps having the reflective coating 3 composed of titanium oxide, apatite or phosphor, have problems which are inherent in the manufacturing steps as mentioned below. The steps for manufacturing conventional fluorescent lamps having a reflective layer are described below with reference to the cross-sectional construction of the conventional fluorescent lamp having a reflective layer as illustrated in FIG. 2.
(1) A step for forming the reflective coating 3 by coating a solution of the abovementioned reflective substances on the inner surfaces of the glass tube 1 at an angle of reflection .theta..
(2) A step for drying the thus formed reflective coating 3.
(3) A step for baking the dried reflective coating 3.
(4) A step for forming a phosphor coating 4 by coating a solution of phosphor on the entire inner surface of the glass tube 1 inclusive of the baked reflective coating 3.
(5) A step for drying the thus formed phosphor coating 4.
(6) A step for baking the dried phosphor coating 4.
According to the conventional art, the fluorescent lamp with reflective layer having the cross-sectional construction as shown in FIG. 2 must be manufactured through the abovementioned six steps. Particularly, the third step (3) for baking the reflective coating 3 markedly disturbs the productivity of the fluorescent lamps having a reflective layer. In other words, the step for baking the reflective coating 3 requires increased periods of time before the fluorescent lamps with reflective layer are completed, increased costs for facilities such as manufacturing machinery, increased areas and space for installing the facilities, and increased labor force. Therefore, the third step (3); presented great difficulty in regard to not only the productivity but also the manufacturing costs of such a lamp. Particularly, the following problem was presented when a specially obtained reflective substance such as titanium oxide or apatite was used as the reflective coating 3. Namely, when such specially obtained reflective substances were used as the reflective coating 3, the abovementioned step (3) had to effect the baking at a temperature considerably higher than the temperature that was used when the phosphor was used as the reflective coating 3, in order to completely remove a binder such as organic solvent or the like. Moreover, the fluorescent lamps employing the specially obtained reflective substances have encountered such a problem that the step for bending the fluorescent lamps into a circular shape was very difficult.